EUV lithography (EUVL) is an emerging technology in the microelectronics industry. It is one of the leading candidates for the fabrication of devices with feature sizes of 70 nm and smaller. Synchrotron radiation facilities provide a convenient source of EUV radiation for the development of this technology.
This invention relates to techniques for generating partially coherent radiation and particularly for converting effectively coherent radiation from a synchrotron to partially coherent extreme ultraviolet radiation suitable for projection photolithography.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a cast image of the subject pattern. Once the image is cast, it is indelibly formed in the coating. The recorded image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent or opaque to the impinging radiation. Exposure of the coating through the transparency placed in close longitudinal proximity to the coating causes the exposed area of the coating to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of 10 to 20 nm) are now at the forefront of research in efforts to achieve smaller transferred feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection (demagnifying) lens onto a wafer. Reticles for EUV projection lithography typically comprise a glass substrate coated with an EUV reflective material and an optical pattern fabricated from an EUV absorbing material covering portions of the reflective surface. In operation, EUV radiation from the illumination system (condenser) is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle rpflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation is re-imaged to the wafer using a reflective optical system and the pattern from the reticle is effectively transcribed to the wafer.
A source of EUV radiation is the laser-produced plasma EUV source, which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000 C, to emit EUV radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line so that malfunction does not close down the entire plant. A stepper employing a laser-produced plasma source is relatively inexpensive and could be housed in existing facilities. It is expected that EUV sources suitable for photolithography that provide bright, incoherent EUV and that employ physics quite different from that of the laser-produced plasma source will be developed. One such source under development is the EUV discharge source.
EUV lithography machines for producing integrated circuit components are described for example in Tichenor et al. U.S. Pat. No. 6,031,598. Referring to FIG. 5, the EUV lithography machine comprises a main vacuum or projection chamber 2 and a source vacuum chamber 4. Source chamber 4 is connected to main chamber 2 through an airlock valve (not shown) which permits either chamber to be accessed without venting or contaminating the environment of the other chamber. Typically, a laser beam 30 is directed by turning mirror 32 into the source chamber 4. A high density gas, such as xenon, is injected into the plasma generator 36 through gas supply 34 and the interaction of the laser beam 30, and gas supply 34 creates a plasma giving off the illumination used in EUV lithography. The EUV radiation is collected by segmented collector 38, that collects about 30% of the available EUV light, and directed toward the pupil optics 42. The pupil optics consists of long narrow mirrors arranged to focus the rays from the collector at grazing angles onto an imaging mirror 43 that redirects the illumination beam through filter/window 44. Filter 44 passes only the desired EUV wavelengths and excludes scattered laser beam light in chamber 4. The illumination beam is then reflected from the relay optics 46, another grazing angle mirror, and then illuminates the pattern on the reticle 48. Mirrors 38, 42, 43, and 46 together comprise the complete illumination system or condenser. The reflected pattern from the reticle 48 then passes through the projection optics 50 which reduces the image size to that desired for printing on the wafer. After exiting the projection optics 50, the beam passes through vacuum window 52. The beam then prints its pattern on wafer 54.
Although no longer under serious consideration for high-volume commercial fabrication applications, synchrotron sources play an extremely important role in the development of EUV lithography technology. Being readily available, highly reliable, and efficient producers of EUV radiation, synchrotron radiation sources are well suited to the development of EUV lithography. These sources are currently used for a variety of at-wavelength EUV metrologies such as reflectometry, interferometry and scatterometry.
In the case of synchrotron radiation sources, there are three types of sources: bending magnets, wigglers, and undulators. In bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted. Wiggler sources comprise a so-called wiggler for the deflection of the electron or of an electron beam. The wiggler includes a multiple number of alternating poled pairs of magnets arranged in a series. When an electron passes through a wiggler, the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further characterized by the fact that no coherency effects occur. The synchrotron radiation produced by a wiggler is similar to that of a bending magnet and radiates in a horizontal steradian. In contrast to the bending magnet, it has a flow that is reinforced by the number of poles of the wiggler.
Finally, in the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and a smaller magnetic field of the deflection pole than in the case of the wiggler, so that interference effects of synchrotron radiation occur. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and radiates both horizontally and vertically in a small steradian element, i.e., the radiation is strongly directed.
In lithographic applications, the partial coherence of the illumination (sigma) is often defined as the ratio of the illumination angular range to the numerical aperture of the imaging (projection optical) system. The illumination angular range is also referred to as the divergence of the source. Undulator radiation is much like a laser source in that it produces highly-coherent light of very low divergence. A typical EUV undulator beamline produces a sigma of less than 0.1 whereas lithographic application nominally require a sigma of 0.7 or higher. Although less coherent than undulator radiation, bending magnet radiation is also typically too coherent to be directly used for lithography.
As EUV lithography technology matures, more lithographic printing stations will be required for resist and process development. Proliferation of these systems has been slowed by the lack of reliable and cost-effective EUV sources. It would be greatly desirable to alleviate the dearth of EUV sources for lithographic process development applications in the form of small-field static microsteppers through the use synchrotron radiation. The relatively high coherence of these sources, however, has precluded them from being used more extensively for actual lithography studies. Relevant process development applications require much more incoherence than is inherently provided by synchrotron sources. This is especially true of undulator sources that otherwise would be highly desirable for their large EUV power capabilities.
The present invention allows the effective coherence of a synchrotron beamline to be tailored to photolithography applications by using a simple single moving element and a simple stationary low-cost spherical mirror. The invention is highly efficient and allows for in situ control of the coherence properties of the illumination.
As stated above, a source with lower coherence has larger divergence, however, simply forcing a coherent illumination source to diverge by way of a focusing optic (i.e. lens, mirror, or Freznel Zone plate) or stationary scatter plate would not actually reduce the beam coherence. Reduced coherence requires that the source divergence be comprised of mutually incoherent propagation angles. When a coherent beam is simply forced to diverge using a static optical element, the resulting propagation angles remain coherent as evidenced by the ability to focus to a near-diffraction-limited spot in the case of a focusing optic or the ability to create speckle as in the case of a scatter plate.
To decorrelate the different propagation angles, and hence decoherentize the radiation, a random time-varying phase term could be imparted to the individual propagation angles or the individual propagation angles could be guaranteed never to coexist in time, thereby ensuring their mutual incoherence. The former case can be accomplished, for example, in the case of a scatter plate by continually moving the plate within the beam. The present invention is based, in part, on the recognition that in the latter case, decoherentizing can be achieved by scanning the beam through a set of angles comprising the desired divergence. The illumination created by such a system will appear to have the coherence dictated by the imparted divergence as long as the observation (exposure) time is made long enough such that the entire range of angles is presented during the exposure.
In one embodiment, the invention is directed to an illuminator device for an optical image processing system, wherein the image processing system comprises an optical system requiring partially coherent illumination, and where the illuminator includes:
a source of coherent or partially coherent radiation which has an intrinsic coherence that is higher than the desired coherence;
a reflective surface that receives incident radiation from said source;
means for moving the reflective surface through a desired range of angles in two dimensions wherein the rate of the motion is fast relative to integration time of said image processing system; and
a condenser optic that re-images the moving reflective surface to the entrance plane of said image processing system, thereby, making the illumination spot in said entrance plane essentially stationary.
In another embodiment, the invention is directed to a method of modifying the coherence of a beam of radiation from a synchrotron source that includes the steps of:
directing the beam of radiation into a reflective surface;
moving the reflective surface through a desired range of angles in two dimensions wherein the rate of the motion is fast relative to the subsequent observation time; and
re-imaging the moving reflective surface to an observation plane, thereby, making the illumination spot in said observation plane essentially stationary.